In potable water membrane purification systems, there are many surfaces that are normally contaminated by bacteria, causing rapid flux decline because of the bacteria themselves, or the slime they produce to attach themselves on surfaces.
It is well known that bacterostatic metal and metal ions can be used to address this problem. The metal and metal ions are chemically or non-chemically bonded to the membrane surface. The action of a bacteriostatic metal, which often includes toxic metals, needs to be on the active high pressure side of the membrane. It is mainly in this region that bacteria can colonize and cause pore blockage.
It is also known that it takes 2-30 ppb of dissolved silver to disinfect 20,000 gallons of water. However, a surface with a bacteriostatic metal e.g. silver and copper, will prevent microbial colonization. The mechanism is not known, but it is possible that even the lowest metal ion concentration (parts per billion) can be detected or xe2x80x9cfeltxe2x80x9d by microorganisms and prevent their attachment and/or growth. It is believed by some that many forms of bacteria, fungus, and virus utilize a specific enzyme for their metabolism. Silver acts as a catalyst, effectively disabling the enzyme. It is toxic to all species tested of bacteria, protazoa, parasites, and many viruses, while copper is specific to fungi. To primitive life forms, silver is as toxic as the most powerful chemical disinfectants. However, this toxicity does not apply to higher life forms, which actually use some heavy metals in their metabolic pathways.
In U.S. Pat. No. 3,953,545, Stoy discloses the use of copper and silver ions as complexes with nitrile groups to be used in the formation of polymers and co-polymers. DuPont discloses in WO 94/15463 the use of barium particles successively coated with silver, copper oxide, silicon, hydrous alumina and droctyl azelate as powders for delustering fibers and providing anti-bacterial properties. A Japanese patent, JP 5,245,349, uses silver on zeolite particles that are added to a membrane formulation. There are many uses of various metals as antibacterial preparations in DuPont U.S. Pat. No. 5,180,585. Nitro, in JP 57084703 and Courtney, Gilehrist, and Park, in GB 1,521,171, disclose a silver xe2x80x9csaltxe2x80x9d or complex similar to U.S. Pat. No. 3,953,545.
Also, U.S. Pat. No. 5,102,547 discloses a synthetic polymer membrane incorporating fine particles of dispersed water-insoluble bioactive material e.g. metal and metal alloys linked to the polymer without chemical binding. Anti-bacterial activity is supposedly provided by the dissolving out of the metal. High metal concentrations of 0.05 to 15%/w, on a dry weight basis, are disclosed. It is noted that the application to water purification is not disclosed, and that in view of the disclosure of the use of several toxic metals antimony, bismuth and mercury, it is apparent that there is no contemplation of water treatment. It is also noted that the loose binding of the metals inside the porous matrix of the membrane instead of the active surface permits dissolution of the metals to the permeate without affecting the concentrate side of the membrane would result in the wastefull washing away of the metal in a water treatment system. This patent could be useful in supplying a source of metal ions to already treated water rather than preventing the fouling of treatment equipment (thus would not prevent fouling and flux decline of the membrane). The present invention, using a reductive technique, places only minute quantities (compared to the whole volume of the membrane) of porous metals such as silver copper and/or nickel at the very point of potential fouling by bacteria. It can also place metal or metal alloys, not only on the membrane surface, but on all surfaces in contact with the water that is being treated. In this way the system can be shut down having to add toxic bacteriostatic chemicals, and without the possibility of bacterial bloom.
To the best of the inventors"" knowledge, there are presently no membranes or systems on the market that are intrinsically bacteriostatic, at the active separation surface layer.
It is an object of our invention to prevent the colorization of membrane surfaces by mircoorganisms when processing water.
The invention relates to a method and apparatus for rendering both systems and individual membranes toxic or otherwise incompatible to pathogens but harmless to animals, including humans. The invention provides a permanent solution to the deleterious effects of bacterial colonization and sliming of membrane systems. The membranes themselves separate all bacteria, viruses and cysts from drinking water in a cost-effective manner. In case adverse conditions affect or deteriorate the bacteriostatic properties of membranes and system, a simple reapplication of the treatment will be able to restore the bacteriostatic properties.
Due to their bacteriorstatic or other desirable properties, silver, copper, tin, nickel and other metals and/or mixtures and alloys are incorporated into a polymer membrane. Several methods can be employed to achieve the desired metal content: (1) adding a metal salt or mixture of compatible salts to the membrane polymer casting solution before formation of the film and gelling it into a bath containing sufficient reducing agent to deposit the metals; (2) once a membrane has been cast in the normal phase-inversion method, performing a static adsorption of soluble metal salts on the polymer membrane followed by exposure to a reducing agent, (3) incorporating the reducing agent into the membrane either as an additive before casting, (4) contacting the cast membrane with reducing agent and then contacting the membrane with the desired salt or mixture of compatible salts that deposit as metals, or (5) contacting the preformed membrane and/or water purification apparatus simultaneously with the metal salts and a reducing agent therefor.
Methods (2), (4) and (5) are preferred, since less metal is used. In Methods (1) and (3), the metal is incorporated in the matrix of the polymer. However, it takes considerably more metal incorporated into the polymer to have the same effect at the surface, as compared to the other methods.
The invention includes the choice of materials that are used and in the method of application.